I don't agree that Christianity was entirely a copy of Egyptian mythology as (nearly) all early religions had elements of astrological notions in them, not just the Egyptian mythos.

The main difference with Christianity (which is really a continuation of Judaism, moving on from the Age of the Ram to the Age of Pisces) is the one God notion, which, was borrowed, in part from a time period in Ancient Egypt when one God was pushed onto populace by the priests.

The Abrahamic religions were well acquainted with their neighboring counterparts and borrowed elements from all of them.